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<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.overview.core.threads"></a><a class="link" href="threads.html" title="Threads and Boost.Asio">Threads and Boost.Asio</a>
</h4></div></div></div>
<h6>
<a name="boost_asio.overview.core.threads.h0"></a>
          <span class="phrase"><a name="boost_asio.overview.core.threads.thread_safety"></a></span><a class="link" href="threads.html#boost_asio.overview.core.threads.thread_safety">Thread
          Safety</a>
        </h6>
<p>
          In general, it is safe to make concurrent use of distinct objects, but
          unsafe to make concurrent use of a single object. However, types such as
          <code class="computeroutput"><span class="identifier">io_context</span></code> provide a stronger
          guarantee that it is safe to use a single object concurrently.
        </p>
<h6>
<a name="boost_asio.overview.core.threads.h1"></a>
          <span class="phrase"><a name="boost_asio.overview.core.threads.thread_pools"></a></span><a class="link" href="threads.html#boost_asio.overview.core.threads.thread_pools">Thread
          Pools</a>
        </h6>
<p>
          Multiple threads may call <code class="computeroutput"><span class="identifier">io_context</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code> to set up a pool of threads from which
          completion handlers may be invoked. This approach may also be used with
          <code class="computeroutput"><span class="identifier">post</span><span class="special">()</span></code>
          as a means to perform arbitrary computational tasks across a thread pool.
        </p>
<p>
          Note that all threads that have joined an <code class="computeroutput"><span class="identifier">io_context</span></code>'s
          pool are considered equivalent, and the <code class="computeroutput"><span class="identifier">io_context</span></code>
          may distribute work across them in an arbitrary fashion.
        </p>
<h6>
<a name="boost_asio.overview.core.threads.h2"></a>
          <span class="phrase"><a name="boost_asio.overview.core.threads.internal_threads"></a></span><a class="link" href="threads.html#boost_asio.overview.core.threads.internal_threads">Internal
          Threads</a>
        </h6>
<p>
          The implementation of this library for a particular platform may make use
          of one or more internal threads to emulate asynchronicity. As far as possible,
          these threads must be invisible to the library user. In particular, the
          threads:
        </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
              must not call the user's code directly; and
            </li>
<li class="listitem">
              must block all signals.
            </li>
</ul></div>
<p>
          This approach is complemented by the following guarantee:
        </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
              Asynchronous completion handlers will only be called from threads that
              are currently calling <code class="computeroutput"><span class="identifier">io_context</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code>.
            </li></ul></div>
<p>
          Consequently, it is the library user's responsibility to create and manage
          all threads to which the notifications will be delivered.
        </p>
<p>
          The reasons for this approach include:
        </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
              By only calling <code class="computeroutput"><span class="identifier">io_context</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code> from a single thread, the user's
              code can avoid the development complexity associated with synchronisation.
              For example, a library user can implement scalable servers that are
              single-threaded (from the user's point of view).
            </li>
<li class="listitem">
              A library user may need to perform initialisation in a thread shortly
              after the thread starts and before any other application code is executed.
              For example, users of Microsoft's COM must call <code class="computeroutput"><span class="identifier">CoInitializeEx</span></code>
              before any other COM operations can be called from that thread.
            </li>
<li class="listitem">
              The library interface is decoupled from interfaces for thread creation
              and management, and permits implementations on platforms where threads
              are not available.
            </li>
</ul></div>
<h6>
<a name="boost_asio.overview.core.threads.h3"></a>
          <span class="phrase"><a name="boost_asio.overview.core.threads.see_also"></a></span><a class="link" href="threads.html#boost_asio.overview.core.threads.see_also">See
          Also</a>
        </h6>
<p>
          <a class="link" href="../../reference/io_context.html" title="io_context">io_context</a>, <a class="link" href="../../reference/post.html" title="post">post</a>.
        </p>
</div>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2021 Christopher
      M. Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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